Docs feedback: Outdated Method

The Install GitLab Runner manually on GNU/Linux document, particularly the Install section's guidance for RPM-based systems is recommending to directly-use the rpm utility for installation-actions rather than the proper, yum- or dnf-based installation method. Direct-use of rpm was appropriate for RHEL 5 and earlier. However, for RHEL 6 and later, yum or dnf are the better tools. The direct-use of rpm will interfere with the ability of yum-based tools to properly track installation and/or updates of the runner RPM.